The present invention relates to a solid state imaging device having an on-chip micro-lens and its manufacturing method.
FIGS. 1, 2A and 2B show an example of a CCD solid state imaging device 1 having an on-chip microlens. This CCD solid state imaging device 1 has a configuration shown in FIG. 1. That is, in its imaging area, a plurality of light receiving sections 2 are arranged in a matrix form. On one side of each column of the light receiving sections 2, a vertical transfer register 4 having a CCD structure is formed via a readout gate section 3. In correspondence with each unit pixel 5, an on-chip micro-lens 6 is formed on its light receiving section 2.
In this example, the unit pixels 5, i.e., their light receiving sections 2 have a pitch in the horizontal direction different from that in the vertical direction. For example, the pitch is coarse in the horizontal direction, and fine in the vertical direction. Assuming now that each unit pixel 5 has a length L.sub.H in the horizontal direction and a length L.sub.V in the vertical direction, the relation L.sub.H &gt;L.sub.V holds true and each unit pixel 5 takes a shape which is long in the horizontal direction.
FIGS. 2A and 2B show sections of FIG. 1 respectively in the horizontal and vertical directions.
On a silicon substrate 12 of a first conduction type, such as, for example, n-type, of the CCD solid state imaging device 1, a first well region 13 of a second conduction type, i.e., p-type is formed. In the first p-type well region 13, an n-type impurity diffusion region 14 and an n-type transfer channel region 16 constituting the vertical transfer register 4 are formed. Although not illustrated, a p-type channel stop region is formed in a pixel separation section 17 as occasion demands. On the n-type impurity diffusion region 14, a p-type positive charge storage area 18 is formed. Just under the transfer channel region 16, a second p-type well region 19 is formed.
By the p-type well region 13, the n-type impurity diffusion region 14, and the p-type positive charge storage region 18, a so-called HAD (hole accumulated diode) sensor is formed. By this HAD sensor, the light receiving section 2 is formed.
Over the transfer channel region 16 constituting the vertical transfer register 4, the readout gate section 3, and the pixel separation section 17, a transfer electrode 22 including a first layer of polycrystal silicon and a second layer of polycrystal silicon is formed via a gate insulation film 21. By the transfer channel region 16, the gate insulation film 21, and the transfer electrode 22, the vertical transfer register 4 of the CCD structure is formed.
On the entire surface including the transfer electrodes 22, an inter-layer insulating film 24 is formed. Furthermore, a light shield film 25 made of, for example, A1 is selectively formed so as to cover a portion of the inter-layer insulating film 24 corresponding to the transfer electrode 22.
In order to obstruct light (light inputted obliquely) inputted from the light receiving section 2 directly to the vertical transfer register 4, a projection 25a partially extending to the side of the light receiving section 2 is formed in the light shield film 25. In other words, an opening section 26 facing the light receiving section 2 is formed in the light shield film 25.
On the entire surface including the light shield film 25, a passivation film 28 is formed. Furthermore, a planarization film 29 is formed. On the planarization film 29, a color filter layer 30 is formed. At a position on the color filter layer 30 corresponding to the light receiving section 2, an on-chip micro-lens 32 is formed.
The color filter layer 30 is formed by a repetition of a pattern having a magenta color 31MG, a cyan color 31Cy, a yellow color 31Ye, and a green color 31G arranged in a rectangle form bisected in both the vertical and horizontal directions. The green color 31G is formed by two layers of the cyan color 31Cy and the yellow color 31Ye.
FIGS. 1 through 4 show a method for manufacturing the conventional CCD solid state imaging device, especially its on-chip micro-lens.
FIGS. 3A, 3B, and FIGS. 5A, 5B are sectional views along lines A--A and B--B, respectively, in corresponding plan views of FIGS. 4 and 6.
First of all, the light receiving section 2, the vertical transfer register 4, the light shield film 25, and the color filter layer 30 are formed as shown in FIGS. 3A, 3B and 4. Thereafter, on the color filter layer 30, a photoresist layer 34 of, for example, a negative type serving as an on-chip lens material is coated and formed via a planarization film 29a. On this photoresist layer 34, a mask 35 is formed. The mask 35 includes a light transmitting section 35a separated so as to correspond to each of pixels, and a light shield sections 35b located between adjacent light transmitting sections 35a. Each light transmitting section 35a takes the shape of a rectangle, or takes a shape which is long in the lateral direction and has circular arcs at both ends in the illustrated horizontal direction. Light is applied via the mask 35, and the photoresist layer 34 is exposed to light.
Subsequently, as shown in FIGS. 5A, 5B and 6, development is conducted so as to remove areas of the photoresist layer which have not yet been exposed to light. Thereafter, heat treatment is conducted at approximately 130 to 200.degree. C. for 30 to 6000 seconds, for example. By using the viscosity of the photoresist, reflow is effected thereon to form a dome-shaped on-chip microlens 32 having a spherical surface. In this way, there is obtained the CCD solid state imaging device 1 having the on-chip micro-lens 32 in correspondence with the light receiving section 2 of each pixel thereon.
In this example, the height between the silicon substrate 12 and the on-chip micro-lens 32 is h.sub.1 in both horizontal and vertical directions. In this example, in order to match focusing in the horizontal direction, the height h.sub.1 between the silicon substrate 12 and the on-chip micro-lens 32 and the thickness of the on-chip micro-lens 32 are determined. In the vertical direction, therefore, focusing is not attained.
In the above described conventional CCD solid state imaging device 1, the unit pixel 5 has a size in the horizontal direction (scanning direction on the television screen) different from that in the vertical direction. In other words, the pixel pitch in the horizontal direction is different from that in the vertical direction. In this case, the single on-chip micro-lens 32 takes the shape of a portion of a spherical surface. Since the curvature in the section of the horizontal direction is different from that of the vertical direction, however, there occurs an extreme difference in focusing efficiency between the horizontal direction and the vertical direction. In other words, if an incident light L is focused in the horizontal direction as shown in FIG. 2A, focusing is not attained in the vertical direction as shown in FIG. 2B, resulting in a lowered focusing efficiency. Therefore, it was difficult to obtain such a focusing state compatible with both the horizontal direction and the vertical direction.
Thus, in order to improve the focusing, it was attempted to narrow the gap between the on-chip micro-lenses 32 at every unit pixel, to form a material having a high refractive index over the opening of the light shield film and under the on-chip color filter so as to be convex downward to be combined with the above-mentioned on-chip micro-lens.
Even in such a case, it was impossible to conduct focusing and propagation of light beyond the focusing efficiency of the on-chip micro-lens. Eventually, the focusing efficiency of the on-chip micro-lens disposed at the uppermost position dominated characteristics such as the sensitivity or the like of the imaging device.
Even in the numerous attempts as described above, a further improvement of the sensitivity becomes a great problem as the pixels of the CCD solid state imaging device become smaller.